With mobile communication systems being applied more and more widely, many of the key techniques become more and more perfect, but there are still some problems needing solution, among which is that of cell handoffs. In a cellular wireless communication system, the coverage area of the wireless communication system consists of numerous partially overlapping cells. When a mobile user is moving in this area, it often occurs that the mobile user connected to a cell handoffs from one cell to another. In the first and second generation cellular mobile communication systems, the frequency used by a specified cell cannot be used again by the adjacent cells, that is, the cells that use a same frequency should have an interval in space between them to ensure frequency multiplexing. Consequently, handoffs of a mobile user between cells are mainly performed between channels with different frequencies, when the handoffs can incur a period of interruption in the voice transmission, leading to the phenomenon of interruption in the communication, and are called hard handoffs.
With the emergence of the third generation mobile communication systems, the soft handoff technique with its advantages in a WCDMA system has increasingly found applications, wherein a user equipment (UE), when establishing a connection with a new cell during movement, does not sever the connection with the original cell immediately, but performs a soft handoff on the same frequency point, thereby reducing the call drop rate significantly. Because the frequency multiplexing factor in a third generation system can reach up to 1, adjacent cells can use a same frequency, making possible soft handoffs between different cells, thus enhancing the communication quality of the entire system.
Similar to a GSM system (Global System for Mobile Communication), a WCDMA system may use multiple frequency points in some user-intensive hotspots to enhance the system capacity, thus forming multiple coverage, and in this case, during the movement of a UE, either variations of the surrounding environment or increase of the number of the users on the currently used frequency point may cause the deterioration of the communication quality of the UE, and only soft handoffs within a frequency may not well satisfy the requirements of the UE on the communication quality, so in areas covered by multiple frequency points, it is necessary to consider employing inter-frequency hard handoffs in order to achieve a better communication quality. Before implementing a hard handoff, it is needed to evaluate comprehensively the communication qualities on different frequency points, and when the conditions are appropriate, handoff the UE to the frequency point with the best quality in time, that is, to perform an inter-frequency hard handoff. Because a hard handoff may possibly cause a call drop, how to reduce the risk of call drops is a key point needing to be considered in a handoff scheme.
A WCDMA system has many its own unique features as compared with other systems such as a GSM system and a CDMA system, and its development period in every countries has been relatively short, so there are not yet many existing technologies that can be tapped on in terms of specific handoff implementations, and in disclosed patents no relevant content on inter-frequency hard handoffs has been found.